At 10 p.m. Sunday, the police and Royal Navy decided to evacuate people within 706 feet of the bomb.

"Overnight, officers have been helping to evacuate properties within the exclusion zone and police are working with the local authority to provide residents with temporary emergency accommodation and the appropriate support," police said.

Some 261 arrivals and departures were scheduled for London City Airport on Monday, according to the BBC.

Authorities from Newham Council say they plan to wait for people to get clear of the exclusion zone before the police and navy begin to lift the device. Then, "when work starts to remove it, it is expected the exclusion zone will be extended to 250 metres [825 feet] and more properties will need to be evacuated."

The council adds that "the operation is expected to continue until Tuesday morning."

Several airlines are moving their flights to other local airports. According to The Associated Press, "regional airline City Jet said its flights from the airport had been rescheduled to land and take off from London Southend airport," and "Italy's Alitalia said it would operate flights from London Stansted airport."

The discovery of World War II era bombs in London is not particularly rare, as NPR's Ari Shapiro has reported. "During the Blitz, German planes dropped nearly 30,000 bombs on London in just three months," he notes.

In 2015, a German bomb of about the same size was discovered in an east London neighborhood, prompting an evacuation.

At that time, Matt Brosnan, a historian at the Imperial War Museum, told the BBC that we don't know exactly how many of the bombs dropped could still be hidden.

"Clearly not all of those would have exploded, because of defects or other reasons, and they could have buried themselves tens of feet below the surface so we simply don't know where they are," he told the broadcaster.

The issue is also not unique to London. As NPR's James Doubek reported last August, 70,000 people evacuated in Frankfurt, Germany, to allow authorities to defuse a British World War II-era bomb.